Charcoal rot a problem for hot, dry beans

In any given year, the big three in soybean diseases in west Tennessee are usually easy to call — frogeye leaf spot, anthracnose and brown spot. But this year’s dry weather created room for another — charcoal rot.

“Charcoal rot has been a big problem in non-irrigated soybeans this year,” said Melvin Newman, University of Tennessee Extension plant pathologist. “It’s taken a lot of yield. It’s taken a plant that is susceptible and weak and taken even more out of it.”

In the future, producers may have more tools at their disposal, including better knowledge of cropping systems that minimize charcoal rot development and resistant or tolerant soybean varieties, according to Alemu Mengistu, USDA/ARS, Jackson, Tenn., speaking at the Soybean Disease, Insect, Weed and Agronomy Field Day, in Milan, Tenn.

According to Mengistu, the disease loves dry conditions and this year’s drought in west Tennessee was perfect for it. Symptoms first appear after flowering when microsclerotia, a paper-like structure, can be seen on the lower part of the stem. “When you split the stem, you see discoloration inside.”

Later on in the season, leaves remain attached to the plant, and they tend to have a darker tint. If you look at the lower part of the stem and the roots, they are all covered with the tiny paper-like structures on the surface and inside the stem.”

Mengistu has developed a rating system to evaluate varieties for resistance to charcoal rot. He grinds the lower stems and roots of soybean plants, places them in a medium, then watches for the development of microbial colonies.

The rating system for resistance is based on how much material has to be ground in order for the colonies to develop. “Using this approach, we were able to identify a resistant variety, DT 97-4290. “It is not immune. It still has some colonies, but it was more resistant than other lines we selected. Moderately tolerant varieties included DP 3478, Hamilton, Jackson II and Asgrow 4715.”

The testing for moderately tolerant lines did not include any Roundup Ready lines, but last year USDA started collaborating with universities to identify charcoal rot resistance in RR varieties. So far, most of the lines looked at are susceptible, although there are a few tolerant lines.

Another approach is to look at how cropping systems affect development of the disease. Mengistu studied the disease’s relationship to no-till and conventional-till cropping systems combined with cover crops. “In some situations, we used cover crops, in some we did not. In some, we used rye, others hairy vetch.”

Mengistu used the colony forming rating to measure the effect of the cropping system on the disease. In 2002, which was a drier year compared with 2003 and 2004, no-till with a rye cover had a lower colony-forming rating than no-till with a hairy vetch cover.

Mengistu noted that there were more organisms recovered in conventional-till than in no-till, but hairy vetch cover still had the highest rating. “This indicates to us that in drier years, we are better off using a crop like rye as a cover crop.”

In the last two years of the study, the differences between the various systems leveled out somewhat because they were drier years relative to 2002, although hairy vetch again showed more organisms in 2003.

Mengistu concluded that “rye could be an alternative cover crop to be considered. We know that hairy vetch is a good legume. It provides nitrogen, but it looks like it is also a susceptible host for this organism. This is something you should consider when managing charcoal rot until we can come up more resistant varieties.”

Mengistu stressed one reason why no-till may produce fewer charcoal rot organisms is because it provides a cooler and more moist environment than conventional tillage.

Researchers are still looking for the mechanism for varietal resistance, according to Mengistu. “Is it genetic? Is it because it has a better taproot and can get moisture?”

Another problem is that the disease has been underestimated in terms of its impact, partially due to a lack of information. Mengistu hopes to remedy that. He also noted that in the past soil sampling was the primary means to determine if charcoal rot organism were present. He says his studies indicate that plant samples “seem to do a better job of detecting it.”